40 Chapter 2 Graphical Summaries of Data As an illustration, Table 2.4 presents the number of visitors, in millions, to several popular websites in February 2009 and in February 2010. Table 2.4 Number of Visitors, in Millions Website February 2009 February 2010 Facebook 65 125 Google 126 137 YouTube 70 93 Microsoft Network (MSN) 37 38 Yahoo 66 66 Source: mostpopularwebsites.net We would like to visualize the changes in the numbers of visitors between 2009 and 2010. Figure 2.3 presents the side-by-side bar graph. The bar graph clearly shows that Facebook’s traffic grew enormously between 2009 and 2010, while the growth in traffic to the other websites was much less pronounced. 140 120 100 80 60 40 20 0 Frequency (millions) Frequency of Visits to Popular Websites 2009 2010 Yahoo Facebook Google YouTube MSN Figure 2.3 Side-by-side bar graph for the number of visitors to various websites Objective 3 Construct pie charts Pie Charts Apie chart is an alternative to the bar graph for displaying relative frequency information.A pie chart is a circle. The circle is divided into sectors, one for each category. The relative sizes of the sectors match the relative frequencies of the categories. For example, if a category has a relative frequency of 0.25, then its sector takes up 25% of the circle. It is customary to label each sector with its relative frequency, expressed as a percentage. Example 2.6 illustrates the method for constructing a pie chart. EXAMPLE 2.6 Constructing a pie chart Construct a pie chart for the computer sales data in Table 2.3. Solution For each category, we must determine how large the sector for that category must be. Since there are 360 degrees in a circle, we multiply the relative frequency of the category by 360 to determine the number of degrees in the sector for that category. For example, the relative frequency for the Desktop category is 0.22. Therefore, the size of the sector for this category is 0.22 · 360◦ = 79◦. Table 2.5 presents the results for all the categories.
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